/* Custom string builder, should be slightly faster than using Sv* macros directly. */

typedef struct {
    SV *sv;
    char *cur;
    char *end;
    size_t maxlen;
} fustr;

/* sv must be a new SV with a preallocated buffer */
static void fustr_init_(pTHX_ fustr *s, SV *sv, size_t maxlen) {
    s->sv = sv;
    SvPOK_only(s->sv);
    s->cur = SvPVX(s->sv);
    s->end = SvEND(s->sv);
    s->maxlen = maxlen;
}

static void fustr_grow(pTHX_ fustr *s, size_t add) {
    size_t off = s->cur - SvPVX(s->sv);
    size_t newlen = 64;
    add += off;
    if (add > s->maxlen) croak("maximum string length exceeded");
    /* Increment to next power of two; SvGROW's default strategy is slow */
    while (newlen < add) newlen <<= 1;
    if (newlen > s->maxlen) newlen = s->maxlen;
    char *buf = SvGROW(s->sv, newlen);
    s->cur = buf + off;
    s->end = buf + (SvLEN(s->sv) > s->maxlen ? s->maxlen : SvLEN(s->sv));
}

static inline void fustr_reserve_(pTHX_ fustr *s, size_t add) {
    if (UNLIKELY(s->end < s->cur + add)) fustr_grow(aTHX_ s, add);
}

static inline void fustr_write_(pTHX_ fustr *s, const char *str, size_t n) {
    fustr_reserve_(aTHX_ s, n);
    memcpy(s->cur, str, n);
    s->cur += n;
}

/* Adds n uninitialized bytes to the string and returns a buffer to write the data to */
static inline char *fustr_write_buf_(pTHX_ fustr *s, size_t n) {
    fustr_reserve_(aTHX_ s, n);
    char *buf = s->cur;
    s->cur += n;
    return buf;
}

static SV *fustr_done_(pTHX_ fustr *s) {
    fustr_reserve_(aTHX_ s, 1);
    *s->cur = 0;
    SvCUR_set(s->sv, s->cur - SvPVX(s->sv));
    // TODO: SvPV_shrink_to_cur?
    return s->sv;
}

#define fustr_init(a,b,c) fustr_init_(aTHX_ a,b,c)
#define fustr_reserve(a,b) fustr_reserve_(aTHX_ a,b)
#define fustr_write(a,b,c) fustr_write_(aTHX_ a,b,c)
#define fustr_write_buf(a,b) fustr_write_buf_(aTHX_ a,b)
#define fustr_done(a) fustr_done_(aTHX_ a)